1. Field of the Invention
The invention relates to an improvement of a device which, when binary image data indicative of existence/nonexistence of dots and stored in a memory in a compressed state are to be transferred to a printer engine of a laser printer or the like, reconstructs the compressed binary image data to the original form.
2. Description of the Related Art
In a digital printer such as a laser printer, image data received from a host computer are first converted into binary image data indicative of existence/nonexistence of dots and then stored in a memory (buffer), and the binary image data are read out from the memory and then transferred to a printer engine which is an execution unit of the printer. Since a printer engine is increasingly improved in printing speed and resolution, such a printer must have an ability of rapidly transferring a large amount of data to the printer engine.
In a high resolution system, it is required to handle a large amount of binary image data. In order to store such a large amount of binary image data in a memory of an appropriately small size, therefore, the binary image data are usually compressed and then stored into the memory. Various kinds of compression methods are known. When compressed binary image data in the memory are read out from the memory and transferred to the engine, the data are reconstructed to the original form.
FIG. 1 schematically shows the configuration of a conventional printer which operates in the manner described above. The printer receives image data from a host computer which is not shown, via an interface circuit 2. The image data are converted into binary image data corresponding to a dot image and then compressed, by a CPU 4. The compressed data are written into an image buffer 10 in a RAM 8. Thereafter, the compressed data in the image buffer 10 are sequentially read out and reconstructed to the original form by the CPU 4. The reconstructed image data are expanded in a print buffer (for example, a band buffer) 12 in the RAM 8. The binary image data in the band buffer 12 are directly read out in a sequential manner by a DMA (Direct Memory Access) circuit 16, and then sent to a smoothing circuit 18. The smoothing circuit 18 corrects the image data so as to smooth unnatural irregularities of outlines of characters such as letters and graphics in the image. The corrected image data are sent to a printer engine 24 via a video interface 22.
In the conventional printer shown in FIG. 1, there frequently occurs a process in which a system bus 14 is occupied for the transmission of image data, with the result that each process consumes a considerable time period, thereby impeding the speed-up of the printer. Specifically, as seen from the above description with reference to FIG. 1, the system bus 14 is occupied for the transmission of image data in all of the following processes: a process in which the printer receives image data from the host computer; a process in which the CPU 4 converts image data from the host computer into binary image data, compresses the binary data, and writes the compressed data into the image buffer 10; that in which the CPU 4 reads out the image data from the image buffer 10, reconstructs the image data, and writes the reconstructed data into the image buffer 12; and that in which the DMA circuit 16 transfers the reconstructed image data in the band buffer 12 to the smoothing circuit 18. Therefore, these processes cannot be simultaneously conducted so that the proportions of the time periods respectively allocated to the processes are inevitably lowered. As a result, the total processing time is prolonged and a high-performance CPU, which is expensive, must be used in order to speed the printer.